A so-called convex type torque converter, in which a radial intermediate part of a front cover is curved toward the outside for improving rigidity of the front cover and forming a space for housing the lockup damper, has conventionally been known (Published Patent Application (KOKAI) No. 59-222624, Published Utility Model Application (KOKAI) No. 59-139652 or Published Patent Application (KOKAI) No. 58-106246 corresponding to U.S. Pat. No. 4,484,898).
For convex type torque converter, two types have been known: a lug fitting type as illustrated in FIG. 8 and a spline fitting type as illustrated in FIG. 9.
However, because it is necessary to form lugs 100 (notches) on a most outer peripheral part of a piston and an outer peripheral part of a damper in the case of FIG. 8, its weight becomes heavy and it is difficulty to reduce cost.
Further, slap of the lug 100 is apt to arise, for example, during idling in the D-range.
Because two plates 101 of the damper are fastened together by rivets 102 the plate thickness can not be reduced and rigidity maintained. Thus, the cost is increased and the weight becomes heavy.
In the case of FIG. 9, it is necessary to form a spline internal tooth 104 on a damper hub 103 and to machine a spline external tooth also on a turbine hub in which the damper hub 103 fits, so that cost and increase in weight become inevitable.